Synchronization system



June 29, E w T R, A D 2,085,424-

SYNCHRONIZATION SYSTEM Filed. Feb. 12, 1956 5 Sheets-Sheet l ANTENNA AN7NNA i A B 1 STATION 6 ANTENNA BUFFER AMI? v l M0 A. F. I I1] V INVENTOR. Map. AEAMR DE wm R.GODDARD ATTORNEY.

June 29; 1937. DE WlTT R. GODDARD 2,085,424

SYNCHRONI ZATION SYSTEM Filed Feb. 12, 1936 3 Sheets-Sheet 2 D/SPZACEMEA/T //V PHASE 0/-' E'W/i'f/ RESPECT 70 F INVENTOR.

DEWITT R.GODDARD ATTORNEY.

June 29, 1937. DE WlTT R. GODDARD 2,085,424

SYNCHRONIZATION SYSTEM Filed Feb. 12, 1936' 3- Sheets-Sheet 3 Jay. 7

AREA a /4.5% AREA b 50% AREA 85.5% AREA 0 /00% mo}: 75% 50% 25% a 1 i +a+ H 6 m0 S T 4 75 w & 5 7055 g. 7085M v E: 0 D

-3so 270 90" 0" +90" H80" +270" +360 DISPZAC'FME/VT IA/ Pf/AJE 0F f'W/TH RESPECT F INVENTOR. DE WITT R. GODDARD ATTORNEY.

Patented June 29, 1937 UNETED STATES SYNCHRONIZATION SYSTEM De Witt Rugg Goddard, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 12, 1936, Serial No. 63,473

2 Claims.

The present invention relates to synchronizing circuits, and more particularly to an arrangement for maintaining synchronism of frequency and/or phase between a plurality of broadcasting stations.

It is known to have two or more broadcasting stations synchronized as to frequency, by employing at each station an accurate frequency controlling element, such as a piezo-electric crystal. It is often required that the carrier frequencies transmitted by the different broadcasting stations be accurately synchronized with respect to one another, so that there is a difference of not more than one-tenth of a cycle in frequency between them. The adjustment of the generated oscillations to obtain synchronism may be made either manually or automatically whenever a frequency difference appears.

The present invention provides an improved circuit arrangement which is sensitive to the phase difference between the carriers of two transmitting stations to maintain the carriers at the same frequency and approximately in phase. Essentially, the invention consists in separately receiving at the control station the two carrier frequencies of the stations it is desired to synchronize and individually beating each of the received waves with locally generated oscillations to produce conveniently low tonal oscillations, and comparing the two locally produced tones by a phase detector to control the phase of the oscillations generated at one transmitter with respect to that of the other.

A feature of the invention is the grid controlled glow discharge device phase detector employed for determining phase differences in the received waves.

A detailed description of the invention follows, together with drawings, wherein:

Fig. 1 shows a schematic drawing of two transmitting stations A and B whose transmitted carriers are accurately synchronized by means of control station C;

Fig. 2 illustrates schematically the circuit arrangement for control station 0;

Figs. 3-8, inclusive, are graphs illustrating the relation between anode current and phase displacement of the phase detector of the system of Fig. 2, and will be referred to in explaining the operation of the phase detector; and

Fig. 9 illustrates another type of phase detector which can be used to replace the special type of phase detector of Fig. 2.

In Fig. 1 there are shown schematically two transmitting stations A and B whose carrier oscillations it is desired to synchronize with respect to one another as to phase and frequency. Each station is provided with an antenna for radiating the carrier oscillations. The apparatus at both of these stations may take any well 5 known form, and is herein shown conventionally in box form. Station C is the control station employing two receivers E and F, each of which has a directive antenna, as illustrated, for separately picking up the radiations from one of the transmitters, either A or B. These receivers, as will appear later from a description of Fig. 2, pass the received energy to a phase detector, and the latter controls the frequency adjusting mechanism of transmitter B over a transmission line D.

Referring to Fig. 2, which shows in detail the apparatus at station 0, above the dot and dash line, and the cooperating frequency adjusting mechanism at station B below the dot and dash 20 line, it will be observed that station C comprises two receivers E and F which have respectively directive antennae E and F. Directive antenna E is pointed at one of the stations A and B and directive antenna F is pointed at the other station for respectively collecting only the energy radiated from that one station toward which it is pointed. The receivers E and F are both fed by a local oscillator O of suitable frequency for producing in their respective outputs a beat signal of suitable low frequency, such as 500 cycles. These two 500 cycle tones in the outputs of receivers E and F are fed to transformers H and K which are in circuit with a special phase detector comprising two gaseous conduction tubes M and N, each of which is provided with a grid GR, an anode AN, and a cathode CA suitably arranged to keep the carrier oscillations of the two transmitters A and B in phase, as will appear more fully hereinafter. Transformers H and K in the outputs of receivers E and F are each provided with two secondary coils, one feeding the grid circuit of one of the grid controlled glow discharge devices while the other coil feeds the anode circuit of the other grid controlled glow discharge device. These devices are known in the art by the trade-name Thyratron and are characterized by the fact that the grid acts as a trigger when supplied with a suitable positive potential, after which the grid loses control and the device continues to produce a flow of current in the anode circuit until such time as the anode current is either cut off or reduced to a potential below a critical value. For a more complete description of the operation of these gaseous conduction tubes, reference is made to an article by Dr. Albert W. Hull, in the General Electric Review for July, 1929, pages 390-399.

Serially connected with the anode of each of the thyratrons M and N is a winding of a polarized relay R. Winding G of the relay is in the anode circuit of device M and winding L in the anode circuit of device N. It should be observed at this time that there are no bias batteries provided for the grids and anodes of the thyratron tubes. The grid resistors Z and W and the anode resistors Z and W prevent the grids and anodes from drawing excessive current during the positive portions of their voltage swings, as will appear more fully later. As long as equal currents flow through these windings, the armature or tongue T of relay R will remain centered, as shown, without engaging either of the two contacts. However, if one of the devices, let us say M, draws less current than the other, let us say N, then tongue T will engage the contact associated with the winding drawing the greater amount of current, in this case L, thus sending a potential, either positive or negative, over the transmission line D, shown in dotted lines, to the frequency adjustment mechanism at the transmitting station B. In the example assumed, a negative potential would be sent over the line D. If, on the other hand, thyratron M has more current flowing in its output winding G than thyratron N, then tongue T will be attracted in the upper direction and a positive potential will flow out over the line D to station B.

The manner in which the thyratrons M and N function is as follows:

Assuming the phase relation indicated by the positive and negative signs on the terminals of the secondary windings of transformers H and K, and that the incoming signals received on antennae E and F are in phase, then thyratron M is subjected to an alternating anode voltage and an alternating grid voltage. Let us examine the instantaneous grid and anode voltages during one complete cycle of the 500 cycle beat notes. As the grid starts to swing from zero volts to ward its maximum negative value, the anode starts to swing from zero volts toward its maximum positive value. During this quarter cycle no anode current can flow as the grid remains negative thereby keeping the anode current cut off. Inasmuch as the anode voltage increases in a positive direction, the grid in turn increases in a negative direction. Now, during the following 90 (or quarter cycle) the anode voltage decreases to zero and the grid voltage likewise decreases to zero and as the grid continues to be negative while the anode is positive no current flows. On the next two quarter cycle periods no anode current can flow as the anode potential swings from zero through a maximum negative value back to zero again. It will thus be observed that, assuming the above phase relations, no anode current will flow at any time. Under the above assumptions, tube N is exposed to the same voltage conditions as thyratron M, and therefore draws no anode current. This is the only condition under which both thyratrons M and N draw no anode current.

Now. let us assume that the transmitter feeding antenna E has drifted a slight amount so that there is a small lag in the phase of the 500 cycle tone feeding transformer H with respect to the 500 cycle voltage at transformer K. With this assumption the following is a description of the behavior of each of the thyratrons:

Considering first tube M, when the anode is at zero voltage starting on its positive swing, the grid has not yet reached zero from its previous positive swing. Therefore when the anode starts on its positive swing the grid is still slightly positive. This is a condition allowing anode current to flow, therefore the anode current starts to flow immediately and continues during the whole of that half cycle irrespective of grid conditions. In other words, the grid loses control once the glow in the gaseous conduction device strikes. When the anode finally reaches zero potential from itspositive swing, the anode current of course stops and cannot flow during the negative anode swing.

The situation for tube N is different. Here, for the same conditions, the grid leads the anode in its phase by a small amount so that as the anode starts from zero voltage towards its positive swing, the grid has already become negative, with the result that no current flows in the anode circuit. However, the grid passes through its negative swing and comes back to zero before the anode has quite completed its positive swing. Therefore, anode current flows for the remainder of its positive swing. During the negative swing of the anode it, of course, passes no current. The total result of this is to make arelatively large average current flow through relay winding G and a relatively small current flow through relay winding L. This would raise the armature R causing it to engage the upper contact sending a voltage of such polarity to the controlled transmitter as to bring it back in phase with the master transmitter.

Fig. 3 shows the behavior of tube M with varying phase displacements between the two transmitters. The abscissa indicates degrees of lead (-1-) or lag of the signal picked up on antenna E with respect to the sign-a1 picked up on antenna F. The ordinate represents the percent of each positive half cycle of the 500 cycle beat note that anode current flows. For instance, suppose the signal picked up on antenna E lags F by 270, then the 500 cycle voltage in-H lags that in K by 270. As the anode of this tube M starts upward on its positive swing, its grid being 270 behind in phase is at the maximum of its negative swing, therefore the tube cannot pass anode current until 90 later or when the grid passes through zero. At this point the anode passes current and continues to pass it for the remaining quarter cycle. That is, the anode passes current of the half cycle positive swing, as shown on the curve in Fig. 2. Once again, letus assume that E lags F by Now, as the voltage on the anode of tube M starts from zero on its positive swing, the grid being 90 behind is at its maximum positive value therefore the anode immediately starts to pass current and continues to do so during the whole of its positive swing. That is, the anode passes current during of its positive half cycle. This value may also be found on the curve of Fig. 3. Of course, no anode current fiows during the negative swing of the anode.

Tube M has been treated with some detail to show how various phase relations of the grid and anode voltages affect the anode current. Tube N is subjected to similar conditions. Due to the fact that the grid and anode voltages are exact-' ly out of phase with the corresponding elements of tube M, the curve of time of flow of plate current versus phase difference of antenna signals assumes a different shape. This curve is shown in Fig. 4. In order to check this curve, let us assume one condition in which E and F are out of phase and trace the behavior of the anode current.

Suppose the signal on antenna E lags behind that of F by 90, then as the anode voltage of tube N passes from a negative value through zero and. starts to rise on the positive half of its cycle, the grid voltage, as it leads the anode voltage by 90, is already around the negative peak of its cycle. Therefore no anode current flows. Now, as the cycle progresses, the grid voltage drops in value to zero. At this point the anode has reached the maximum of its positive swing and, since the grid voltage is zero, breaks down and anode current flows. It continues to flow during the remaining 90 of its positive swing. In other words, anode current fiows during of the positive half of one cycle of the 500 cycle tone. This value may be read from Fig. 4 by spotting a point on the curve directly over 90 and projecting this point horizontally to the ordinate scale. This ordinate value is 50% as computed above.

Fig. 5 shows both curves superimposed. It is evident from this figure that there are two values of phase displacement that can be used to control the adjustable transmitter. One is at zero degrees and the other is at 180. The 180 setting has been assumed as the more desirable as the greater the phase displacement from this value, the greater the correcting force on the relay GL. If this setting is used, then the setting at zero phase displacement will give reverse control. This offers no handicap as the controlled transmitter would merely be kept off exact synchronism until the phases once more passed through 180 relation at which point the control would take hold again.

So far the foregoing exposition has dealt with phase angle between the two receiving antennas and time of flow of anode current. Now, as the anodes of the thyratrons are subjected to an alternating current of some wave form, their instantaneous anode currents do not conform to the curves shown in Fig. 3 and Fig. 4. It is probably worth while to look into this as, of course, it is the anode current of the tubes that actuates the relay GL.

If it is assumed that the anode voltage has a sinusoidal wave form, then the curve in Fig. 3 may be redrawn to show what the average anode current of tube M will be for all values of phase displacement between the alternating currents present in transformers H and K. This curve is shown in Fig. 6. It is obtained by plotting average anode current against phase displacement. That is, turning to Fig. 3, at 90 the anode conducts 100% of its positive half cycle, therefore the average anode current is 100% of the area under one-half of a sine wave, see Fig. 7. This gives point i on the curve in Fig. 6. Point 2 has the same value. At -225, Fig. 3 gives a value of 75%, that is, the anode is passing current for 75% of its positive half cycle. Referring to Fig. 6, the area under 7 5% of the half sine wave shown is 85.5%. This gives point 3 on the curve in Fig. 6. In a similar manner, points 4 and 5 are found. Fig. 8 shows a composite curve comprising the anode current characteristic of both thyratrons with respect to phase displacement of the twoincoming signals.

In describing the behavior of thyratrons M and N of Fig. 2, there have been assumed two conditions for the sake of simplicity. First, that any negative potential on the grids of the thyratrons will keep the anode cut ofi" and secondly, that with the grids at any positive potential, anode current will start to fiow as soon as any positive potential is applied to the anodes. The latter assumption is not exactly true as it requires about +15 volts to set up ionization within a thyratron. This fifteen volts becomes a negligible factor if the voltages applied from transformers H and K are high. The fact that the anode may swing a few volts positive without anode current flowing assists in correcting the first erroneous assumption, for under certain conditions described above, it allows the grid to assume a negative potential of suflicient magnitude to prevent anode current from flowing before the anode voltage has reached a high enough voltage to start anode current.

The polarity sensitive frequency corrector for station E, shown in Fig. 2, below the dot and dash lines, is merely illustrative of any type of apparatus which may be used for effecting the frequency adjustment. In the illustration, there is shown a motor Q which may be operated in either of two directions, depending upon the application of the polarity to the line D to adjust a variable condenser U for regulating the frequency of the oscillator Y, which is controlled as to frequency by a frequency controlling element such as a piezo-electric crystal PE. The output of oscillator Y feeds any suitable type of transmitting circuit arrangement through a blocking condenser X. The transmitting circuit shown comprises a buffer amplifier BA feeding a power amplifier PA in whose output circuit is an antenna. Associated with the power amplifier is a modulator circuit MO which is responsive to the energy impressed on the microphone V and audio frequency amplifier AF for modulating the output of the power amplifier. The invention is not limited to any one type of frequency correction circuit since, if desired, other types of frequency correcting mechanism may be used, such as are described in United States Patent No. 2,027,196, granted January 7, 1936, to Arthur Pfister. A particularly desirable type of electro-mechanical drive for condenser V is described in my copending application Serial No. 11,915, filed March 20, 1935.

Fig. 9 illustrates another type of phase corrector which can be used instead of that shown in Fig. 2. This corrector dilfers from that of Fig. 2 mainly in the use of high vacuum tubes M, N and the provision of transformers H, K which have single secondary windings. The two tubes M and N, in the manner illustrated in the drawings, constitute a pair of differential detectnrs. The electromotive force of given frequency coming in over transformer K is combined with the output electromotive force from transformer H on the grid circuits of the vacuum tubes M, N. Under normal conditions the two electromotive forces superposed in the grid circuits will be 90 apart, and the electromotive forces on the two grids of the tubes M and N will be of equal magnitude, and since the two tubes have a grid battery I adjusted to make detectors of the tubes, the output currents in the relay windings G and L will be equal and the relay armature T will stand open at neutral positions.

But, if the relative phase relations of the alternating electromotive forces from H and K de-' part from 90, then the currents in the relay windings G and L will become unequal and the armature T will go to one or the other of its adjacent contacts to effect frequency correction via line D in the same way as explained for Fig 2.

What is claimed is:

1. The method of synchronization which consists in producing two radio waves whose frequencies are to be synchronized, radiating said waves, separately receiving each of said radiated waves, and separately beating said received waves with a third wave to obtain resultant beat notes of audible frequencies, and controlling the frequency of one of the waves to be synchronized in accordance With the difference between said audible frequencies.

2. A phase detector comprising a first transformer and a second transformer each having a single primary and two secondary windings, a first and second gaseous conduction device each having an anode, a cathode, and a grid, means for connecting the grid and cathode of said first device across one of the secondary windings of said first transformer, and means for connecting the grid and cathode of the other device across the correspondingly located secondary winding of the second transformer, and means for conmeeting the cathode and anode of the second device to the other secondary winding of the first transformer, and the anode and cathode of the first device to the other secondary Winding of the second transformer, a polarized relay having two windings, one of which is serially arranged in the anode circuit of one device and the other of which is serially arranged in the anode circuit of the other device, said relay having an armature and two oppositely disposed contacts arranged to be alternately engaged by said armature, means for applying a positive potential to one of said contacts and a negative potential to the other of said contacts, two sources of frequency which are to be synchronized coupled to the primary windings of said transformers, and means connected to the armature of said polarized relay for correcting the frequency of one of N said sources to be synchronized.

DE WITT RUGG GODDARD. 

